1. Field of the Invention
The invention relates to a storage rack for the storage of fuel elements of nuclear reactors consisting of a sheet metal lattice arrangement constituting a plurality of abutting similar vertical storage cases or tubes having in general a rectangular cross section. In anyone of the cases of this storage rack a rod-shaped fuel element may be stored.
2. Description of the Prior Art
Within or in the neighbourhood of the reactor building there are available storage pools for storing spent or new thermal reactor fuel elements during a long or only a short period. Therein the fuel elements are placed in racks below water in such an arrangement that the necessary heat dissipation is warranted and no nuclear chain reaction may occur. Generally for such a design there are included very broad margins with respect to the mutual distances between the elements. When the storage capacity of such pools has been exhausted and the transfer of the spent fuel elements to a regeneration plant is not yet possible a temporary solution is found by placing the rods closer to each other in the pool in which case however, much more attention should be given to warrant sufficient criticality margins for the ensemble.
A possibility for reducing the mutual distance or the pitch with which the fuel elements are being placed is the inclusion in the construction of a so-called neutron poison, i.e. a material having a very high effective cross section value for the absorption of neutrons. Such a material is the boron isotope B.sup.10 that may be included as an alloying component in the stainless steel, mostly used for the manufacture of these racks. When utilising such a material for the manufacture of the storage racks as a whole or part thereof the possibility arises of a much smaller pitch of the fuel elements and consequently of a much more compact storage than in the conventional racks.
From nuclear physic calculations it is apparent that dependent on the chosen construction, the geometry of the ensemble and the specific fuel element, there may be found an optimum ratio of the construction material and the surrounding water with respect to the fuel elements. In case of the storage racks of the above-mentioned type there exists therefore a certain interstice between the cases determining the final storage capacity.
At the found optimum any further increase of the amount of boron, i.e. the increase of the thickness of the borated sheet steel of the cases does not lead to an increase of the absorption capacity and consequently not to a decrease of the water gap or the mutual distance between the elements; that is to say that when the "saturation value" of the boron content is reached a decrease of the mutual distance between the elements will bring the ensemble closer to the criticality. Hence the effective multiplication factor will more closely approach the limit value usually assumed at 0.95.
In case of storage rack arrangements including a neutron poison the water gap between the fuel rods play an important role as a moderator for the neutrons leading to a relatively high peak of the neutron flux between the said cases. This "surge" of the neutron flux at the location of the gap and consequently also in the case walls of borated steel leads to an increased capture of neutrons and consequently to a decrease of the multiplication factor of the ensemble.
Also in case of a rack not provided with a neutron poison a decrease of the mutual distance between the elements leads to an increase of the multiplication factor after all.
In case of a given rack construction and given fuel elements a theoretical minimum distance or gap width constituted by metal and water should be maintained between the fuel elements in order to satisfy the criterion for the multiplication factor.
However, theoretical gap widths may only be applied when the storage rack may be manufactured within very close tolerances.
Up till now these storage racks have been made as a welded construction. Notwithstanding the use of welding jigs the accurate gage maintenance and the attainable straightness of the cases and other construction parts constituting the storage rack are limited by the deformations inherent to each welding process. Therefore in case of a welded construction the theoretically permissible minimum gap width has to be increased with additional safety margins in view of the necessary manufacture tolerances.